In chemical looping combustion, oxygen is transferred from an air reactor to fuel reactor utilizing an oxygen carrier, typically a metal oxide. The oxygen earners are reduced and fuel is oxidized in the fuel reactor, after which the reduced oxygen carriers are returned to the air reactor for regeneration to an oxidized state. With this arrangement, the fuel and the air remain separated, and the process generates a stream of oxygen depleted air leaving the air reactor and a stream of combustion gases, mainly CO2 and H2O, leaving the fuel reactor. After condensing he water, relatively pure CO2 is obtained from the exhaust gas of the fuel reactor. Correspondingly, CO2 separation is inherent to the process.
When gaseous fuels are utilized in chemical looping combustion systems, the fuel is introduced into the fuel reactor as a reducing gas, and appropriate reaction between the oxygen carrier and the reducing gas becomes largely a function of facilitating germane thermodynamic conditions. However, in contrast to these gaseous fuel approaches, significant differences arise when utilizing a solid carbonaceous fuel such as coal, coke, coal and biomass char, and the like. Since fossil carbon largely occurs as a solid rather than as or liquid, it is desirable to adapt the chemical looping combustion process to solid fuels. Generally, this is accomplished by either directly introducing the solid fuel into the fuel reactor, or by first gasifying the solid fuel in a primary step to generate a gaseous fuel, following gaseous fuel chemical looping combustion. The latter method is less desirable due to the necessity for a separate, preliminary gasifier.
When solid fuels are directly introduced into the fuel reactor of a chemical looping combustion system, fuel conversion generally proceeds through drying, devolatilization, and gasification, described in reactions (1) and (2) below:Solid fuel→volatiles+char  (1)C+H2O→CO+H2   (2)
Depending on the type of oxygen carrier utilized, such as Fe2O3, gases produced by devolatilization and gasification can interact with and be oxidized by the oxygen carrier according to:CnH2m+(2n+m)MexOy→nCO2+mH2O+(2n+m)MexOy-1   (3)C+H2O/CO2→CO+H2/CO   (4)H2/CO+MexOy→H2O/CO2+MexOy-1   (5)
Alternatively, an oxygen carrier such as CuO may be utilized which releases gaseous oxygen in the fuel reactor according to:2MexOy→2MexOy-1+O2   (6)
Following the applicable reactions above in the fuel reactor, the reduced oxygen carrier is then oxidized in the air reactor according to:O2+2MexOy-1→2MexOy   (7)
When utilizing directly introduced solid fuels, the applicable fuel reactor reactions are typically generated by fluidizing both the fuel and the oxygen carrier in the fuel reactor, which can generate several significant issues typically not present when gaseous fuels are utilized. For example, with both oxygen carrier and fuel particles present in the fuel reactor, there is significant risk that char may follow the oxygen carrier particle flow and proceed to the air reactor, where it burns and produces CO2. Consequently, char separation between the fuel reactor and the air reactor is necessary. Typically a carbon stripper must be utilized specifically for this purpose. See e.g., U.S. Pat. No. 7,767,191 to Thomas et al., issued Aug. 3, 2010; U.S. Pat. No. 7,824,574 issued to White et al., issued Nov. 11, 2010; U.S. patent application Ser. No. 13/375,957 by Beal et al., Pub. No. US 2012/0167808 published Jul. 5, 2012; and U.S. patent application Ser. No. 13/272,647 by Cao et al., Pub. No. US 2012/0124106 published Aug. 23, 2012, among others. Additionally, the solid fuel generates ash in the fuel reactor, and interaction between ash particles and the oxygen carrier leads to gradual deactivation of the oxygen carrier. See e.g., Siriwardane et al., “Chemical Looping Combustion of Coal with Metal Oxide Oxygen Carriers,” Energy & Fuels 23 (2009); see also Wang et al., “Mechanistic investigation of chemical looping combustion of coal with Fe2O3 Oxygen Carrier,” Fuel 90 (2011), among others. Further, when the ash is periodically discharged from the fuel reactor, oxygen carrier particles may be elutriated in the ash stream and lost from the system, and similar inactivation and separation issues may be expected with the additional use of oxides, halides, and carbonates as char gasification catalysts in the fuel reactor. Thus, for chemical looping processes utilizing solid fuels, the system must be optimized to obtain satisfactory separation between the oxygen carrier and both the char and ash produced in the fuel reactor.
It would he advantageous to provide a fuel reactor for a chemical looping combustion system which allowed various fuel reactor reactions to occur in a manner which maintained separation between the oxygen carrier and the solid fuel during the reactions. Such a fuel reactor would greatly mitigate the additional issues associated with solid fuel chemical looping combustion, such as the necessity for carbon stripping, detrimental oxygen carrier—ash interactions, and loss of the oxygen carrier with the ash stream. Such a fuel reactor would further allow for the addition of catalytic materials to increase and manage solid fuel gasification while minimizing impact on the oxygen carrier.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.